char takes 4 bytes?

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originally posted by mountain8848@Nov 9 2005, 10:10 AM

char test_char=-5;
printf("test_char %x,%x,%x\n",test_char,(char)test_char,(unsign char)test_char);

we expect the output was:

>test_char fb,fb,fb

but we got :

>test_char fffffffb,fffffffb,fb

why dose "char" variable have 4 bytes?

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The char is converted to int by the printf. since 0xfb is a negative number, the resulting value is sign extended to 4 bytes, that is, 0xf...ffb

bye

Paolo

Hi, paolo

Look the below codes, we got the result what we not expected, either.

char test_char=-5;
unsigned char good=0xfb;
if(test_char==good){
  printf("they are equal\n");
}else{
  printf("they are not equal\n");
}
if(test_char==0xfb){
  printf("test_char is 0xfb\n");
}else{
  printf("test_char is not 0xfb\n");
}

the result:

>they are not equal

>test_char is not 0xfb

what should we do? As we should modify lots of codes what these error.

Mountain,

Mask it with 0xff, this will "cut" off the upper 24 bits.

if((test_char&0xff)==good)

Doug

Hi All,

> what should we do?

Start by writing good code ... pick your data type and stick with it.

The result of relational and equality operators is of type int when the operands are

integral types ... so, char and unsigned char are promoted to type int (the range of

both can be represented by an int) ... and the sign is preserved (or "extended" into

the higher-order bits).

Basically, if (for whatever reason) bit 7 of your 8-bit data is not a sign bit, then

don&#39;t treat it as such -- use unsigned char instead (and ditto for short and

unsigned short) ... you&#39;ll save yourself some headaches and you can use:

if(test_char==good)

with confidence.

Regards,

--Scott